Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in what is called a projection exposure apparatus, which comprises an illumination device and a projection lens. The image of a mask (=reticle) illuminated by way of the illumination device is in this case projected by way of the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In the lithography process, undesired defects on the mask have a particularly disadvantageous effect since they can be reproduced with every exposure step. In this case, undesired defect should be understood to mean any deviation of the mask structure from the ideal design that leads to the structure being transferred to the wafer in a manner deviating from the targeted manner. In order to minimize the mask defects and in order to realize a successful mask repair, a direct and fast analysis of the imaging effect of possible defect positions is thus desirable.
For such ascertainment of defects and also in further applications for characterizing the structures on the mask both with regard to existing deviations of the respective structure from the targeted position predefined in each case by the design (so called positioning error or “registration error”, known as: “Registration”) and with regard to the linewidth of the structures (CD=“critical dimension”), in the art methods are known in which a reference image used for the respective characterization (e.g. for defect inspection or position determination) is generated by simulation.
In this case, it is known, in particular, to implement said simulation as rigorous simulation. Such a rigorous electromagnetic simulation involves describing the interaction of the light field with the mask whilst taking account of the three-dimensionality of the mask and also the dielectric properties thereof and the electromagnetic interface conditions prevailing at the respective surface, wherein the three-dimensional geometry and also the concrete layer structure of the mask are taken into account. Furthermore, polarization effects (describable by Jones matrices) of the mask and also of the optical imaging in the optical system (e.g. the position measuring device) are also taken into account. Implementing rigorous simulations has the advantage of a significantly higher accuracy in comparison for instance with so-called Kirchhoff simulation (=scalar approximation), in which all effects associated with the three-dimensionality of the mask are disregarded and which becomes increasingly faulty in particular for structures of the order of magnitude of the optical wavelength or in the case of polarization effects.
One problem that occurs here in practice, however, is that as the complexity of the masks used in microlithography increases, the implementation of rigorous simulations over the entire mask or for all of the mask structures situated thereon leads to expenditure of time and computational complexity that are no longer tenable. In this case, inter alia, so called auxiliary structures having structure sizes below the resolution limit of the respective optical system also contribute to the complexity of the mask, and although they are not themselves imaged onto the wafer in the lithography process, they are required in order to realize a desired imaging of the mask structures onto the wafer (e.g. in order to reduce so called “optical proximity” effects).
On the other hand, however, a transition to approximative methods for the purpose of limiting the expenditure of time and computational complexity during reference image generation necessarily leads to correspondingly great inaccuracies and hence a possibly erroneous characterization of the mask.
With regard to the prior art, merely by way of example, reference is made to U.S. Pat. No. 8,918,743, B1, US 2004/0122636 A1 and DE 10 2011 078 999 A1.